FEATURE NANOTECHNOLOGY
ENERGY HARVESTING for portable applications
Tony Armstrong, Director of Product Marketing at Linear Technology Corporation investigates how nanopower conversion is proving ideal for energy harvesting in wireless sensor networks
here is plenty of ambient energy in the world around us and the conventional approach for energy harvesting has been through solar panels and wind generators. However, new harvesting tools allow us to produce electrical energy from a wide variety of ambient sources. Furthermore, it is not the energy conversion efficiency of the circuits that is important, but more the amount of “average harvested” energy that is available to power it. For instance, thermoelectric generators convert heat to electricity, Piezo elements convert mechanical vibration, photovoltaics convert sunlight (or any photon source) and galvanics convert energy from moisture. This makes it possible to power remote sensors, or to charge a storage device such as a capacitor or thin film battery, so that a microprocessor or transmitter can be powered from a remote location without a local power source. Nevertheless, it is at the “low” end of the power spectrum, where nanopower conversion in WSNs (Wireless Sensor Networks) is becoming more common, where the need for power conversion ICs which can work with very low levels of power and current are needed the most. These are often 10s of microWatts and nanoAmps of current, respectively. However, the availability of such power conversion products, including battery charges, operating at sub-1µA of current are extremely limited. WSNs are basically a self-contained system consisting of some kind of transducer to convert the ambient energy source into an electrical signal, usually followed by a DC/DC converter and manager to supply the downstream electronics with the right voltage level and current. The downstream electronics consist of a micro- controller, a sensor and a transceiver. When trying to implement WSNs, a good question to consider is: how much power do I need to operate it? Conceptually this would seem fairly
T 10 SUMMER 2015 | MICROMATTERS
complexity since they must now take into consideration how much energy must be stored in the secondary reservoir to compensate for the lack of an ambient energy source. It is clear that WSNs have very low
Figure 1:
The LTC3388-1/-3 is a 20V input capable synchronous buck converter for charging and protecting Lithium- ion/Polymer batteries
“The
LTC4071 is a shunt battery charger system that includes integrated battery pack protection and a low battery disconnect feature to protect low capacity batteries ...”
straightforward; however, in reality it is a little more difficult due to a number of factors. For instance, how frequently does a reading need to be taken? Or, more importantly, how large will the data packet be and how far does it need to be transmitted? This is due to the transceiver consuming approximately 50% of the energy used by the system for a single sensor reading. Several factors affect the power consumption characteristics of an energy harvesting system of WSN. Of course, the energy provided by the energy harvesting source depends on how long the source is in operation. Therefore, the primary metric for comparison of scavenged sources is power density, not energy density. Energy harvesting is generally subject to low, variable and unpredictable levels of available power so a hybrid structure that interfaces to the harvester and a secondary power reservoir is often used. The harvester, because of its unlimited energy supply and deficiency in power, is the energy source of the system. The secondary power reservoir, either a battery or a capacitor, yields
higher output power but stores less energy, supplying power when required but otherwise regularly receiving charge from the harvester. Thus, in situations when there is no ambient energy from which to harvest power, the secondary power reservoir must be used to power the WSN. Of course, from a system designer’s perspective, this adds a further degree of
levels of energy available. This, in turn, means that the components used in the system must be able to deal with these low power levels. While this has already been attained with the transceivers and microcontrollers, on the power conversion and battery charging side of the equation, there has been a void. However, Linear Technology has developed its LTC3388-1/-3 and LTC4071 to address these requirements. The LTC3388-1/-3 is a 20V input capable synchronous buck converter that can deliver up to 50mA of continuous output current from a 3mm x 3mm DFN package. It operates from an input voltage range of 2.7V to 20V, making it ideal for a wide range of energy harvesting and battery-powered applications including “keep-alive” and industrial control power. The LTC4071 is a shunt battery charger system that includes integrated battery pack protection and a low battery disconnect feature to protect low capacity batteries from damage due to self- discharge. It is a simple, yet sophisticated charger and protector for Lithium- ion/Polymer batteries. Its ultra-low 550nA operating current enables charging from previously unusable very low current, intermittent or continuous charging sources such as that supplied from energy harvesting applications. An internal thermal battery conditioner reduces the float voltage to protect Li-Ion/Polymer cells, coin cells or thin film batteries at elevated battery temperatures. This device provides a complete and ultra-compact charger solution with just a single external resistor required in series with the input voltage. Even though portable applications and energy harvesting systems have a broad range of power levels for their correct operation, from microWatts to greater than 1W, there are many power conversion ICs available for selection by the system designer. However, it is at the lower end of the power range, where the levels fall into the nanopower level that the choice becomes limited. Fortunately, there are power conversion and battery charging solutions available for the designer to select from with quiescent currents of less than a microAmp to prolong battery life for keep-alive circuits in portable electronics.
Linear Technology (UK) Ltd.
www.linear.com 01628 477 066
/ MICROMATTERS
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112